FIG. 1 shows the structure of a circuit for one pixel section (pixel circuit) of a basic active organic EL display device, and FIG. 2 shows the structure, and input signals of, a display panel.
A data signal is written to a storage capacitor C by setting a gate line (Gate) that extends in the horizontal direction to a high level to turn an n-channel selection TFT 2 on, and in this state placing a data signal (image data) having a voltage corresponding to a display brightness on a data line (Data) that extends in the vertical direction. In this way, a gate of a p-channel drive TFT 1 is set to a voltage corresponding to the data signal, drive current corresponding to the data signal is supplied to the organic EL element 3, and the organic EL element 3 emits light.
Pixel data, a horizontal sync signal (HD), a pixel clock and other drive signals are supplied to a source driver 10. The pixel data signal is sent to the source driver 10 in synchronism with the pixel clock, held in an internal latch circuit once a single horizontal line of pixels have been acquired, and subjected to D/A conversion all at once to supply to a data line (Data) of a corresponding column. Also, the horizontal sync signal (HD), other drive signals and a vertical sync signal (VD) are supplied to a gate driver 12. The gate driver 12 performs control to sequentially turn on gate lines (Gate) arranged horizontally along each line, so that image data is supplied to pixels of the corresponding line. The pixel circuit of FIG. 1 is provided in the pixel sections 14 that are arranged in a matrix shape. Also, a power supply line PVDD is arranged in the vertical direction along a pixel row, and CV is connected to a power supply CV with anodes of the organic EL element provided common to all pixels.
As a result of this type of structure, data are sequentially written to each pixel in horizontal line units, and display is carried out at each pixel in accordance with the written data, to perform image display as a panel.
Here the amount of light emission and current of the organic EL element 3 are in a substantially proportional relationship. Normally, a voltage (Vth) is supplied across the gate of the drive TFT 1 and PVdd such that a drain current approaching that for a black level of the pixel starts to flow. Also, the amplitude of the image signal is an amplitude so as to give a prescribed brightness close to a white level.
FIG. 3 shows a relationship for current “CV current “(corresponding to brightness) flowing in the organic EL element with respect to input signal voltage (voltage of the data line Data) of the drive TFT 1. It is possible to carry out appropriate gradation control for the organic EL element by determining the data signal so that Vb is supplied as the black level voltage and Vw is supplied as the white level voltage.
Specifically, the brightness when the pixel is driven at a particular voltage differs depending on the threshold voltage (Vth) of the drive TFT, and an input voltage close to PVdd (power supply voltage)−Vth (threshold voltage) corresponds to a signal voltage when displaying black. Also, the slope (μ) of the V-I curve of a TFT varies in a similar manner, and in this case, as shown in FIG. 4, an input amplitude (Vp−p) for outputting the same brightness is also different.
If there are variations in Vth and μ of the TFT inside the panel, there will usually be inconsistencies in brightness. With the objective of correcting these brightness inconsistencies, panel current flowing when lighting up each pixel at a number of signal levels is measured, to obtain a V-I curve for individual TFTs.
The related art suffers from the following problems.
1) According to the prior art, image current is measured in real time during image display. In order to suppress effects of aging of the drive TFTs and organic EL elements, it is necessary to measure the current of each particular specified pixel independently. However, during image display, current corresponding to a display image constantly flows in from a power supply, which shows that it is difficult to measure current of a particular specified pixel by measuring power supply current of the panel.
2) FIG. 1 is one example of a pixel circuit, but in actual fact, as shown in FIG. 5, there are distributed constant circuits on each power supply line and signal line due to wiring resistance and stray capacitance etc. Specifically, there are RC distributed constant circuits in the data lines Data between the source driver 10 and the drain of the select TFT 2, in the gate lines Gate between the gate driver 12 and the gate of the select TFT 2, in the power supply lines between the power supply PVDD and the source of the drive TFT 1, and between the cathodes of the organic EL element 3 and the power supply CV.
Therefore, in the event that a voltage is supplied from outside in order to measure PVDD or CV current, the measurement current gradually increases. Accordingly, it is necessary to perform measurement of current with current at a sufficient level of stability, but the fastest measurement time is determined by this, and a comparatively long time is taken to measure one pixel current. A relationship between current Id flowing in the organic EL element and the current Ipvdd flowing from the power supply PVDD to the drive TFT is shown in FIG. 7. In this way it takes a longer time for current Ipvdd to become stable compared to Id. CV current is also subject to the effects of the distributed constant circuits and can be considered to vary similarly to Ipvdd.
3) Only a single pixel is lit at the time of current measurement, but a very small leakage current also flows in pixels that are not lit. Leakage current is generally extremely small, but since leakage currents for a number of pixels (the number of panel pixels−1) are summed, it becomes a value that cannot be ignored. In particular, in the event that leakage current varies over time, it constitutes a noise component, which shows that measurement accuracy will be affected.